Prepare the filling first: divide the Anko into small balls about the size of a ping-pong ball (or smaller, depending on how big you want each daifuku). Smooth each ball and set on a tray, then chill briefly in the refrigerator so they hold their shape while you work.
In a microwave-safe or heatproof bowl, combine the Shiratamako with the Sugar and Water. Stir until the mixture is smooth and lump-free — it should have the consistency of heavy cream at this stage.
Cook the dough mixture until it becomes translucent and very sticky: if using a microwave, cover loosely and heat in 30–45 second bursts, stirring and checking between bursts, until glossy and elastic (usually 1½–2 minutes total depending on power). If steaming, steam for 10–12 minutes, then knead briefly until smooth.
Prepare a work surface by dusting it generously with Potato Starch to keep the mochi from sticking. Transfer the hot dough onto the dusted surface (be careful, it will be hot) and dust your hands with more starch.
Portion the mochi into pieces slightly larger than your filling balls. Flatten each piece into a round disc, place a chilled Anko ball in the center, then gently stretch and wrap the mochi around the filling, pinching the seams closed and shaping into a smooth ball. Re-dust with Potato Starch as needed to prevent sticking.
Once all daifuku are assembled, let them rest at room temperature for 20–30 minutes to allow the texture to stabilize. Serve within a day or two for best texture; store loosely covered at room temperature or refrigerated briefly (bring back to room temperature before eating).
